1. Field of the Invention
The present invention relates to forwarding data packets towards a mobile node roaming in a foreign packet-switched data network, the data packets having been addressed to a home address of the mobile node, the mobile node being provided with a care of address for use in the foreign packet-switched data network. In particular, but not exclusively, the present invention relates to Mobile IP.
2. Description of the Related Art
Whereas conventional 2 G mobile networks, such as those conforming to the Global System for Mobile Communications (GSM) standards, have provided circuit-switched voice and data services to user's mobile stations (MSs), there is great momentum in the mobile telecommunications industry to deploy packet-switched mobile networks. Packet-switched mobile networks have significant advantages in terms of network and radio resource efficiency and also enable the provision of more advanced user services. With the convergence of fixed and mobile telecommunications, the Internet Protocol (IP), widespread in fixed networks, is the natural choice as the packet routing mechanism for mobile packet networks. Currently IP version 4 (IPv4) is in widespread use in the fixed network domain. However, it is expected gradually to migrate to IP version 6 (IPv6) which offers well-recognized benefits over IPv4, notably in terms of greatly increased address space, more efficient routing, greater scalability, improved security, Quality of Service (QoS) integration, support for multicasting and other features.
A particular example of a mobile packet-switched service currently being deployed is the General Packet Radio Service (GPRS) as implemented in both 2 G GSM networks and in 3 G Universal Mobile Telecommunications System (UMTS) networks (hereinafter referred to as GPRS networks). It is also expected that non-GPRS wireless access technologies, such as wireless Local Area Network (wLAN), will provide a flexible and cost-effective complement to GPRS for local broadband service access in some areas such as hotspots (conference centers, airports, exhibition centers, etc). wLAN subnetworks may be implemented within the same administrative network domain as GPRS subnetworks, and mobile network operators will want to support mobility of mobile stations between those subnetworks. Furthermore, mobile network operators will want to support roaming of mobile stations between different administrative network domains, which may or may not implement different access technologies.
While GPRS networks, having been designed from the start as mobile networks, have built-in mobility management (for MS s within the GPRS network) and roaming functionality (for MSs roaming between GPRS networks), work has also taken place in the Internet Engineering Task Force (IETF) to support mobility of IP user terminals in general. To this end, the IETF have developed the Mobile IP (M P) protocols. MIP is designed to support mobility when mobile stations (or mobile nodes (MNs) in MIP terminology) move between IP networks with different subnet prefixes (macro-mobility). For example, MIP may be used to support mobility between a GPRS network and a non-GPRS network such as a wLAN network as well as mobility between two different GPRS networks or subnetworks. Mobile IP is not expected to be used for mobility management within a network or subnetwork (micro-mobility) which is typically managed by access technology specific layer 2 mechanisms such as WCDMA softer/soft handover.
There are two versions of MIP to correspond to the two versions of IP. MIP version 4 (MIPv4) is designed to provide IP address mobility for IP version 4 (IPv4) addresses, whereas the newer MIP version 6 (MIPv6) MIP is designed to provide IP address mobility for IP version 6 (IPv6) addresses. MIPv4 is described in the IETF Request For Comment (RFC) 3344 available at the IETF website. Internet draft MIPv6 is described in the IETF Internet draft “Mobility Support in IPv6” available at the IETF website.
Mobility management as defined in MIPv4 is illustrated in FIG. 1. A MN 10 is allocated a home IP address (HAddr) in its Home Network (HN) 12. Routing procedures in the HN ensure that wherever the MN is within the HN, an IP packet sent from a Correspondent Node (CN) 16 will reach the MN. However, when the MN roams to a foreign network (FN) 14, IP packets addressed to its HAddr will need to be routed to its new location in the FN. In MIPv4, a router 18 in the HN known as the Home Agent (HA) is used to act as a packet forwarding service on behalf of the MN when it is away from home. In a first working mode of MIPv4 (known as FA-CoA mode), when arriving in the FN, the MN is allocated a Care of Address (CoA) by a router 20 in the FN known as the Foreign Agent (FA). Due to perceived limitations of IPv4 address space, it is envisaged that more than one MN may share the same CoA. After allocation of the CoA, MN 10 sends a binding update message 22 to the HA via the FA to register the CoA. More specifically, the binding update message 22 informs the HA of the association (or binding) between the HAddr and CoA of the MN. Thereafter, when the CN sends a packet to the HAddr of the MN in its HN (case 1), the packet is intercepted by the HA and tunnelled to the FA in the FN via tunnel 24 on the basis of the CoA.
Tunnelling involves encapsulating a first data packet (with a header and a payload) as the payload of a second data packet having a new header indicating, as its source and destination addresses, the start and end points of the tunnel, and transferring the second data packet as normal to the tunnel endpoint where it is decapsulated to obtain the first packet. After decapsulation, the tunnel end point, the FA, routes the original packet to the MN using routing procedures in the FN. In MIP, tunnelling involves IP in IP encapsulation using the IETF Request For Comment (RFC) 2003. Thus in MIPv4, an IPv4 packet is tunnelled by encapsulating it within another IPv4 packet.
As an optional procedure in MIPv4, the HA may send a binding update message 26 to the CN to register the CoA of the MN. More specifically, the binding update message 26 informs the CN of the association (or binding) between the HAddr and CoA of the MN. Thereafter, the CN may address packets directly to the MN at its current CoA, rather than indirectly via its HAddr (case 2), and these packets are received by the FA in the FN and routed to the MN using routing procedures in the FN. This is known as route optimization since it avoids potentially inefficient triangular routing via the HA which in general will not be on an efficient routing path between the CN and the FA.
In a second optional working mode of MIPv4 (known as CoCoA mode) there is no sharing of CoAs by MNs away from their home network and no FA is used. The MN is allocated a unique CoA, known as a co-located CoA (CoCoA). In this working mode, the MN sends a binding update message directly to its HA to register its newly allocated CoCoA. Thereafter, packets sent by a CN and addressed to the MN at its HAddr are tunnelled from the HA directly to the MN. As with FA-CoA mode, as an optional procedure in CoCoA mode, the MN may also send a binding update to a CN to register its CoCoA. Thereafter, packets may be sent by the CN directly to the MN at its CoCoA.
Mobility management as defined in MIPv6 is illustrated in FIG. 2. Two notable differences of MIPv6 over MIPv4 are as follows. Firstly, due to the greatly increased address space in IPv6, CoAs allocated to a MN in a FN are never shared (i.e., they correspond to the optional CoCoA in MIPv4). Secondly, as a result, there is no need to deploy a FA in the FN. Referring to FIG. 2, with MIPv6, when a MN 10 moves from its HN 12 to a FN 14, it is allocated a unique CoA and sends a binding update message 28 directly to its HA 18 in its HN to register the CoA. Packets from a CN 16 addressed to the HAddr are intercepted by the HA 18 (case 1) and tunnelled to the CoA via tunnel 30. This tunnelling may be achieved using IPv6 Generic Packet Tunnelling Mechanism described in IETF RFC 2473. However, in MIPv6, route optimization is not an option but a fundamental part of the protocol and, in general, the MN (not the HA as in MIPv4) should send a binding update message 32 to the CN so that it may address packets directly to the MN at its CoA (case 2). When an MN receives a packet tunnelled from a CN via the MN's HA, it may take this as an indication that the CN has no binding for the MN and initiate a CN binding update.
Both MIPv4 and MIPv6 allow more than one router in each network or subnetwork to act as a mobile agents (i.e., a HAs or FAs) serving the population of MNs. In other words, MIPv4 and MIPv6 allow for redundancy of mobile agents. Mobile agent (MA) discovery mechanisms are provided so that individual MNs may identify an appropriate router to act as their HA or FA for a period of time. Thus, MIPv4 and MIPv6 allow for load balancing and provide a degree of reliability by protecting against the vulnerability of having a single router acting as a MA and thereby being a central point of failure.
However, reliability is still expected to be a problem due to the nature of the functions performed by MAs in both MIPv4 and MIPv6. Also, scalability and network efficiency are expected to be problematic. The problems are expected to be most acute with MNs potentially widely distributed over various networks or subnetworks covering large geographical areas.